penis vs pecker

penis and pecker both are nouns.

Word	Noun	Adjective	Verb	Adverb
penis	Yes	No	No	No
pecker	Yes	No	No	No

As nouns, pecker is a hyponym of penis; that is, pecker is a word with a more specific, narrower meaning than penis:

Other hyponyms of penis include cock, dick, prick, putz, shaft, tool, micropenis, microphallus, erection, hard-on.

penis (noun)	pecker (noun)
the male sex organ	bird with strong claws and a stiff tail adapted for climbing and a hard chisel-like bill for boring into wood for insects
	horny projecting mouth of a bird
	obscene terms for penis